The present invention relates to a clock spring used in automobile steering systems or the like which comprises a stationary member and a movable member which are electrically connected to each other by using cables.
Clock springs are generally used as electrical connectors, for example, in automobile steering systems or the like and comprise a stationary member and a movable member rotatably mounted relative to the stationary member, these members being electrically connected to each other by using flexible cables.
A known example of such a clock spring comprises a cable receiving portion provided between a stationary member and a movable member, and a flat cable which is formed by laminating a plurality of conductor strips between two band films and which is wound into a coil and gently received in the cable receiving portion, one end of the flat cable being fixed to an outer ring wall formed in either the stationary member or the movable member, the other end being fixed to an inner ring wall formed in the other member.
This clock spring permits the flat cable received in the cable receiving portion to be wound on the inner ring wall and rewound on the outer ring wall in correspondence with the rotational direction of the movable member. Thus, substantially no tension acts on the flat cables between a state where the flat cable is completely wound on the inner ring wall and a state where the flat cable is completely rewound on the outer ring wall. It is therefore possible to constantly maintain the electrical connection between the stationary member and the movable member both of which rotate relative to each other.
In the above-described clock spring, since the flat cable is wound and rewound by employing a difference between the diameter of the outer ring wall and that of the inner ring wall, when the rotational amount of the movable member is constant, the length of the flat cable, that can be used, can be reduced by increasing the difference between these two diameters. However, the diameter of the inner ring wall is generally determined by the diameter of a rotational shaft, for example, a steering shaft of an automobile car, in which the clock spring is installed. On the other hand, since the diameter of the outer ring wall can be only slightly increased because there is a demand for reduction in the size of the clock spring, the difference between the diameter of the outer ring wall and that of the inner ring wall can also be only slightly increased. Such a clock spring therefore has a problem in that it generally requires a long flat cable, and, as is already known, the use of a long flat cable causes difficulties as regards the production of the clock spring and thus causes an increase in the total cost thereof.